Show me the money
Just like Hungary and Slovakia, Poland is already a fully paid-up member of the EU. Despite that, the Polish currency is still the Zloty, and will be for a couple of years yet. One Zloty (abbreviation = PLN) equals 100 Groszy, but don't worry woo much about them. The current exchange rate is around six Zloty to one of your British Pounds. And one of your Pounds is equivalent to one frothy Polish beer.
If you come armed with Pounds or Euros, you'll have no trouble finding somewhere to exchange them (look for the word "kantor", that's Polish for exchange office), but your best bet is to just bring your cashpoint card. The ATM's all over Poland are not only convenient and easy to find, you'll also get the best exchange rate. Moreover, if you're using a popular credit card like American Express or VISA, you'll have great success waving them around in restaurants or bars (just check for the relevant sticker on the door first). On all accounts, steer clear of the money changers that pester people in touristy areas and train stations - they are illegal and you may end up with much less money than you started with.
Average costs
(at time of writing)
Poland is still one of the cheapest countries in the Central European region, just look at these prices:
-one beer (6 PLN = 1.25 GBP)
-one Marlboro (6 PLN = 1.25 GBP)
-decent wine (15 PLN = 3 GBP)
-lunch (30 PLN = 6.25 GBP)
-taxi (approx. 2 PLN per km = 0.42 GBP per km)
A word on tipping
Giving 10-15% to waitresses, waiters and hotel staff is a nice gesture. They won't spit in your food or beat you up if you don't, it's just the done thing. You won't miss an extra couple of quid anyway - you're on holiday.
Links:
Stag weekends in Poland (Krakow)
Stagweekends in Poland (Warsaw)

